Gas springs (also known as Thermo Dynamic Elements, TDEs) are used in sliding gate valve mechanisms which control the flow of molten steel during the continuous casting process. The springs within the mechanism are used to clamp face-to-face relatively sliding refractory plates together. The plates each have a bore in the middle; when the two bores are aligned the flow of steel is at its maximum. Depending on the position of these plates the aperture of the bore increases or decreases thereby determining the flow rate of the steel.
The springs must supply sufficient force to ensure that molten steel cannot get between the plates (which is known as finning), which could then potentially lead to a breakout of molten steel, which can be extremely dangerous as well as costly. The sliding of the refractory plates is powered by hydraulics.
A gas spring (TDE) consists essentially of a metal ram connected to a metal bellows (usually stainless steel), with an inert gas in a sealed chamber and applying pressure to the ram. In one form the gas is inside the bellows which thus expand to extend the ram as the gas expands; in another form the bellows are sealed into an outer container and the gas is outside the bellows and ram and inside the container, thereby to compress the bellows and extend the ram as the gas expands.
The springs need to be of compact design whilst providing adequate force. Gas springs have proven capable of satisfying this requirement.
Examples are shown in GB-A-1 457 708.
During the casting process the temperature near the spring pockets can exceed 400° C.
Gas springs have the advantage that at higher temperatures the gas pressure increases to apply a greater force on the sliding plates, thereby preventing any finning. They also provide an extra element of safety, since the normal failure mode is due to the bellows fatiguing. As such, it would be extremely unlikely for all the gas springs within the mechanism to fail at the same time, unlike other types of spring which may have a failure mode related to temperature and thus may all fail together at the same temperature.
Known designs of gas spring can provide a guaranteed number of heats (cycles) at a maximum temperature of e.g. 500° C. However, there have been many cases where the gas springs have been operating in temperatures above 700° C.
For certain mechanisms the force applied by the gas spring at high temperatures is too great to allow the sliding mechanism to operate.